The present invention relates to a fuzzy inference-based digital control method and its apparatus and in particular to a fuzzy inference-based digital control method and its apparatus useful to process control applications.
With the development of digital computers, automated control using DDC (direct digital control) has become popular. However, manipulation conducted by human operators could not be automated.
In recent years, however, fuzzy control using a fuzzy proposition (a proposition which is fuzzy in correctness such as the proposition that "X is large") and an implication rule (IF-THEN statement) to represent the operator's thinking and to use them as control rules has been attempted.
A practical technique of inference in this fuzzy control and generation of fuzzy control rules are described in detail in Japanese paper, "Measurement and Control", vol. 22, No. 1, pp. 84 to 86.
The basic concept of fuzzy control is disclosed in L.A. Zadeh, "Outline of a New Approach to the Analysis of Complex Systems and Decision Process", IEEE Trans. SMC-3, 28/44, 1973. The definition of terms used herein conforms to this paper.
Application of this concept to the process is disclosed in P.M. Larsen: "Industrial Application of Fuzzy Logic Control", Int. J. Man-Machine Studies, 12, 3-10, 1980 and L.G. Umbers and King: "Analysis of Human Decision-Making in Cement Control and the Implications for Automation", Int. J. Man-Machine Studies, 12, 11-23, 1980, for example.
In case of the prior art as disclosed in the aforementioned Japanese Paper, the expressible inference rule is limited to an inference rule of recognition-action type based on an IF-THEN statement of a single step expressed as "IF the measurement is A THEN output B".
However, the inference performed by the operator usually comprises two steps of:
(i) Estimating the value of an attribute of a controlled object represented by unmeasurable variables (such as material jam, a value of which has an expression like large or small) in accordance with a certain model of the controlled object on the basis of measured values of the pressure, temperature, water quality or the like in the process control; and PA1 (ii) deriving the output value to the controlled object by applying a relationship defined on the basis of an empirical rule onto the resultant estimated attribute value. PA1 (iii) IF the measured value is A THEN the state variable value is C; and PA1 (iv) IF the state variable value is D THEN output B. PA1 (1) The conclusion which should be originally obtained by chaining inferences (in two steps in case of the above described (i) and (ii)) is expressed by inference of a single step. Therefore, the logical linkage between the first supposition (IF .about.) and the last result (output .about.) becomes obscure. That is to say, the reason or guarantee why the conclusion is obtained from the supposition becomes obscure. Tuning of the inference rule and verification of the correctness thus becomes difficult. PA1 (2) The number of inference rules is increased.
In such a case, respective inference rules can be represented as:
These expressions do not conform to the expression method of the prior art. By examining the input-to-output relations of the inference rules having forms (i) and (ii), therefore, those inference rules must be united into an IF-THEN statement of a single step with the state variables omitted.
However, such unification causes problems as follows: